Efficient architecture for 3D and planar ultrasonic imaging - synthetic axial acquisition and method thereof

ABSTRACT

An improved device and method for collecting data used for ultrasonic imaging. The data is gathered over numerous transmit and echo receive cycles, or iterations and combined into a synthetic acquisition representing a complete echo characteristic acquisition. At each iteration, only a portion, or subset, of the echo characteristic is sampled and stored. During the iterations, the portion of the echo characteristic that is measured and sampled is varied by changing the relative sampling instants. That is, the time offset from the transmission to the respective sampling instant is varied. The sample sets representative of the entire echo characteristic are then compiled from the multiple subsets of the ultrasonic transmissions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 60/616,176, filed Oct. 5, 2004, entitled “Efficient Architecture for3D and Planar Ultrasonic Imaging—Synthetic Axial Acquisition and MethodThereof”, which is hereby incorporated by reference herein in itsentirety.

This application is also related to International Application No.PCT/US03/06607, filed Mar. 6, 2003, entitled “An Intuitive UltrasonicImaging System and Related Method Thereof,” and corresponding U.S.patent application Ser. No. 10/506,722 filed Sep. 7, 2004 of which areassigned to the present assignee and are hereby incorporated byreference herein in their entirety. The present invention may beimplemented with the technology discussed throughout aforementionedInternational Application No. PCT/US03/06607 and U.S. patent applicationSer. No. 10/506,722.

The present application is also related to PCT International ApplicationNo. PCT/US04/00888, filed Jan. 14, 2004, entitled “Ultrasonic TransducerDrive,” of which is assigned to the present assignee and is herebyincorporated by reference herein in its entirety. The present inventionmay be implemented with the technology discussed throughoutaforementioned International Application No. PCT/US04/00888.

The present application is also related to PCT International ApplicationNo. PCT/US04/00887, filed Jan. 14, 2004, entitled “Ultrasound ImagingBeam-former Method and Apparatus,” of which is assigned to the presentassignee and is hereby incorporated by reference herein in its entirety.The present invention may be implemented with the technology discussedthroughout aforementioned International Application No. PCT/US04/00887.

The present application is also related to PCT International ApplicationNo. PCT/US2004/001002, filed Jan. 15, 2004, entitled “EfficientUltrasound System for Two-dimensional C-Scan Imaging and Related Methodthereof,” of which is assigned to the present assignee and is herebyincorporated by reference herein in its entirety. The present inventionmay be implemented with the technology discussed throughoutaforementioned International Application No. PCT/US04/US2004/001002.

FIELD OF THE INVENTION

The present invention relates to ultrasound imaging devices andtechniques. More specifically, the various embodiments of the presentinvention provide a novel beamforming strategy and system architecturethat may be used to form either 2D C-scan images or 3D images using a 2Dtransducer array. Additionally, the system and methods may be used toform b-scan images, and A-Mode data.

BACKGROUND

Medical imaging is a field dominated by high cost systems that are socomplex as to require specialized personnel for operation and theservices of experienced physicians for image interpretation. Medicalultrasound, which is considered a low cost modality, utilizes imagingsystems costing as much as $250K. These systems are operated bysonographers with several years of training or specialized physicians.This high-tech, high-cost approach works very well for criticaldiagnostic procedures. However it makes ultrasound impractical for manyof the routine tasks for which it would be clinically useful.

The block diagram of a conventional phased array ultrasound system 10 isshown in FIG. 1. A piezoelectric transducer array 12 (or usingalternative electrical/ultrasound transduction mechanism—e.g. capacitivemicro-machined devices), shown on the left, acts as the interface to thebody by converting electrical signals to acoustic pulses and vice versa.Image formation begins when the states of the transmit/receive switches(TX/RX switches 14) are altered to connect the transducer elements toindividual transmit generators TX 16. The transmit generators 16 outputtime varying waveforms with delay and amplitude variations selected toproduce a desired acoustic beam. Voltages of up to approximately 150Volts are applied to the transducer elements. Once transmission iscomplete, the state of the TX/RX switch 14 is changed to connect thereceive circuitry to each element. Incoming voltage echoes are amplifiedby preamplifiers (preamp 18) and Time Gain Control (TGC 20) circuits tocompensate for signal losses associated with diffraction andattenuation. Next, sample and hold circuits (S/H 22) and analog todigital converters (A/D 24) digitize the signals. Finally, the signalsare dynamically delayed and summed within one or more custom integratedcircuits 26 to yield a single focused Radio Frequency (RF) echo line.This signal forms the basis of one image line.

While conventional beamforming approaches produce high quality images,they also impose significant restrictions on the use of ultrasound. The40 Msample/s S/H and A/D circuits employed by these systems, and thehigh data rates they engender, result in high system cost andcomplexity. A modern state-of-the-art imaging system may cost as much as$250,000 and require weeks or months of user training to produce thehighest quality images. Furthermore, while the transducers used by thesesystems are typically only a few centimeters on a side, the electronicsrequired to form images resides in a box with dimensions on the order of2′×3′×4′. Thus, while ultrasound systems are certainly portable, theyare far from the scale that would allow each clinician to carry one in apocket.

The applicability of conventional ultrasound is further limited by thetypical image format used. Images are produced in what is commonlyreferred to as a B-Mode format, representing a tomographic slice throughthe body perpendicular to the skin surface. This image format isnon-intuitive and the act of mentally registering the B-Mode image tothe patient's anatomy requires significant experience.

Significant reductions in system cost and complexity have occurred overthe last five years. Some of the more notable advances have beendemonstrated by Sonosite. Its most recent product, which sells forapproximately $12,000, produces B-Mode and color flow images using ahand-held system. This system produces good quality images and willcertainly broaden the applications for ultrasound in medicine.Unfortunately, the Sonosite architecture and strategy does not appear tobe capable of extension to real-time 3D imaging. Furthermore, the B-Modeimage format produced by the Sonosite system and all other conventionalsystems is not intuitive to most first-time ultrasound users. Noviceusers often have difficulty mapping the image displayed on the screen tothe tissue lying beneath the transducer. This most likely results fromthe distance (a few feet) and orientation differences between the targetand image.

Ultrasonic imaging has the potential to be a common component of nearlyevery medical examination and procedure. But the complexity and expenseof the existing ultrasound systems are an impediment to its widespreaduse. Consequently, an improvement is desired.

SUMMARY

The present invention provides an improved device and method forcollecting data used for ultrasonic imaging. The data is gathered overnumerous transmit and echo receive iterations and combined into asynthetic acquisition representing a complete echo characteristicacquisition. At each iteration, only a portion, or subset, of the echosignal is sampled and stored. During the iterations, the portion of theecho characteristic that is measured and sampled is varied by changingthe relative sampling instants. That is, the time offset from thetransmission to the respective sampling instant is varied. The samplesets representative of the entire echo characteristic are then compiledfrom the multiple subsets of the ultrasonic transmissions. Thus, ratherthan obtaining an entire set of echo samples needed for image processingfrom each echo, the method described herein performs syntheticacquisition via data collection iterations by transmitting an ultrasonicpulse, and receiving a pulse echo at a plurality of array elements, andat each element, obtaining a pulse echo sample subset at a respectivetime offset, wherein the time offset is varied. Then, for each of therelevant array elements, the sample subsets are used to compile a singlesynthesized composite pulse echo sample set that is representative of anecho characteristic at that element at a desired effective samplingrate. In one embodiment, the pulse echo sample subset contains a singlesample. The single sample may be a real valued sample, or may be acomplex valued sample obtained by sampling the output signals from ademodulation circuit. Complex samples can also be approximated bycombining two real samples separated by a quarter period of the echosignal's center frequency. This is referred to as “Directly SampledIn-phase and Quadrature”, or DSIQ (See K. Ranangathan, M. K. Santy, J.A. Hossack, T. N. Blalock and W. F. Walker ‘Direct sampled I/Qbeamforming for compact and very low cost ultrasound imaging’, IEEETransactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol.59, No. 9, pp. 1082-1094 (2004), the contents of which are incorporatedherein by reference).

Alternatively, the subset may be a plurality of samples. A subset havinga plurality of samples may be obtained by full-rate sampling of the echosignal for a portion of the echo. In this embodiment, each subsetcontains no more than half the desired samples. In one embodiment, eachsubset contains no more than six consecutive samples.

In an alternative embodiment, the plurality of samples in each subsetmay be obtained by sampling at a rate lower than the desired effectivesampling rate, and then interleaving the samples from multiple subsets.In some embodiments the desired effective sampling rate may be muchhigher than the Nyquist rate, and even the lower-rate sampling may begreater than the Nyquist rate. But in alternative embodiments operatingat a desired effective sampling rate of one or two times the Nyquistrate, this likely will result in sub-Nyquist rate sampling to obtaineach subset. That is, samples may be obtained from a single echo signalat rate below the Nyquist rate. Certainly this is the case where asingle sample per transmit event is obtained. In subsequent iterations,samples (also at a sub-Nyquist rate) are obtained, where the samplinginstants have a different time offset.

In both cases (successive sampling or sub-Nyquist sampling), the subsetsare then combined to obtain to a full-length sample set representing theecho characteristic, sampled at an effective rate of at least theNyquist rate. For embodiments using successive sampling, the samplesubsets are concatenated into a single record for each element, whereasfor embodiments using sub-Nyquist sampling, the sample subsets areinterleaved to obtain the full echo characteristic sample record. Thebeginning and ending time offsets are determined in large part by thetissue depth of the desired image.

In some preferred embodiments, every element in the array is used toobtain sample subsets. In other embodiments, only some of the arrayelements are used to obtain samples during a given iteration. Forexample, in some embodiments, the pulse echo sample subsets are obtainedfrom shared sampling hardware resources. In one such embodiment, asingle analog to digital converter is shared between two or moreelements. In this case, more than one transmit pulse is required inorder to obtain one subset for each array element. In this case, thenumber of array elements used in a given iteration depends on how manyelements are assigned to a single converter. This is also referred to asa hardware reuse factor. For example, a hardware reuse factor of twomeans that two elements share resources, and thus one half the elementsare able to obtain sample subsets during any given iteration.

The step of aggregating the pulse echo sample subsets of varied offsetsinto a single synthesized composite pulse echo sample set may beperformed by concatenation or interleaving, as described herein, or itmay also include manipulating the sample subsets in response to at leastone motion estimation value as well as the respective time offsets usedto gather the data.

In this regard, one or more array elements may be used to obtain samplesused for motion estimation. A single motion estimate may be obtainedfrom individual direct-sampled complex echo samples taken over aplurality of iterations from a single element, each sample having thesame time offset. Alternatively, multiple motion estimation values maybe derived from samples taken from elements that are at differentlocations within the array. In this manner, the motion may be evaluatedfor different regions of the array to accommodate tipping of the arrayas might occur when pressed against the tissue to be imaged. These maybe referred to as regional motion estimation values. The regional motionestimation values may then be used to generate array-element specificmotion estimation values.

In an alternative embodiment, echo sample subsets that overlap in timemay be gathered at each iteration. That is, the time offsets at eachiteration are adjusted such that the relative sampling instants havesubstantial overlap. In this way, the synthesized echo record may becompiled from the subsets, and in addition, consecutive echo samplesubsets may be used to estimate motion. This data collection mechanismmay be used at a single array element, regional array elements, or allarray elements.

However obtained, the motion estimation value(s) may be used toenable/disable image processing to prevent the display of distortedimages caused by excessive motion. Alternatively, the motion estimationvalue(s) may be used to alter or manipulate the data samples whenforming the synthesized echo response characteristic in order tocompensate for the motion.

One preferred method described herein performs multiple transmit firingsof ultrasonic waveforms from an ultrasonic transducer array oftransducer elements; after each transmit firing, a subset of echosamples of an ultrasonic echo is obtained at each of a plurality oftransducer array elements, wherein each subset is a portion of a desiredecho response as determined by a time offset; an equivalent full set ofecho samples is retrospectively formulated from the subsets for each ofthe plurality of transducer array elements.

Application of the methods described herein will allow useful ultrasoundsystems to be produced at an extremely low cost. By eliminating many ofthe custom integrated circuits used in conventional systems and byaltering the beamforming algorithm to enable implementation in off theshelf programmable DSP chips, this will also dramatically reduce thesize of imaging systems. The potential impact in health care is broadand significant. We have identified a few possible clinical applicationsof this approach, although we believe that others will become apparentas clinicians gain access to the device.

These as well as other aspects, advantages, and alternatives will becomeapparent to those of ordinary skill in the art by reading the followingdetailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art transducer array and datasampling hardware.

FIG. 2 is a block diagram depicting functional components of atransducer and associated sampling hardware resources arranged toimplement an exemplary embodiment of the invention.

FIG. 3 is a block diagram depicting an alternative embodiment of thesampling hardware to obtain either approximate complex samples (DSIQ) ormultiple samples per subset.

FIG. 4 is a block diagram depicting an alternative embodiment of thesampling hardware wherein the hardware resources are shared among aplurality of elements.

FIG. 5 is a block diagram depicting an alternative embodiment of thesampling hardware wherein the hardware resources are shared among aplurality of elements, and the sampling hardware may obtain eitherapproximate complex samples (DSIQ) or multiple samples per subset.

FIG. 6 is a block diagram of an exemplary embodiment of a simplified A/Dconverter.

FIG. 7 is a timing diagram showing various embodiments of the samplingoperation.

FIG. 8 is a flow diagram depicting one preferred method of theinvention.

DETAILED DESCRIPTION

With respect to FIG. 2, a device 200 used to gather data over numeroustransmit and echo receive cycles, or iterations, is shown. Array 202 oftransducer elements provides the transmit and receive interface to thetissue. A single column of elements 204, 206, 208, 210 is shown, withthe understanding that all elements are similarly connected torespective circuitry. Isolation circuit 212 provides isolation of thereceive circuitry, and may take the form of isolation switches, asdescribed with respect to FIG. 1. (Note that the transmit generators arenot shown). The transducer elements preferably provide an unfocusedplanar ultrasound wave propagating into the object being imaged. Asdescribed in one or more of the related applications identified above,this strategy advantageously results in a simplified hardwareconfiguration. In addition, receive focusing alone can achievereasonable spatial and contrast resolution, while preserving anacceptable signal to noise ratio (SNR).

However, alternative embodiments may utilize conventional transmit beamfocusing techniques, such as using time varying waveforms with delay andamplitude variations selected to produce a desired acoustic beam. Inpreferred embodiments using transmit beam focusing, a relatively smallnumber of relatively wide transmit foci are used so as to cover therequired field of view with a small number of discrete transmit foci. Inalternative embodiments where wide transmit foci are not used, the timeof acquisition becomes larger, and may result in a lower frame rate.

Each receive channel has a preamplifier 214 that is preferably impedancematched to the transducer element to maximize sensitivity and bandwidth.An additional amplifier 216 takes the place of the TGC used byconventional systems. Because only a subset of samples of the echowaveform is obtained per each transmit event, the rapid gain adjustmentstypical in TGC circuits are unnecessary, and may thus be optionallyomitted. Next the signal enters S/H circuits 218, 220, 222, 224 whichsample data over a few nanoseconds (ns) and hold their outputs for aslong as one millisecond (ms).

The sampling instants are determined by timing circuit 236. In theembodiment shown in FIG. 2, each receive channel operates on the sametiming signal, and hence obtains samples at the same time offset. Thatis, the sampling instant is the same across all elements, and is variedfrom iteration to iteration. Preferably, the earliest sampling instantis determined by the depth of the target to be imaged, and the samplinginstant is delayed for each successive iteration by increasing the timeoffset by a single sampling interval. The outputs are then digitized byanalog to digital (A/D) converters 226, 228, 230, 232. Sampling may thusbe performed once per transmit event.

In some embodiments, it may be desirable to obtain an image where theC-plane is not parallel to the transducer face. In this case, the roundtrip times to the various locations on the target image plane arecalculated and used as the basis of the time offsets (i.e., samplingdelays) used to capture the records. In this case, the time offsetsprovided by timing circuit 236 are adjusted accordingly, and are not thesame for all transducer array elements, although they may be the samefor each row or column, depending on the orientation of the desiredC-plane to the transducer array. Thus, in this case, the initialsampling instant may be adjusted for each element, as well as theparticular sampling instants.

Finally, the samples are provided to the synthesizer 234, where thesamples are compiled and used to formulate a complete set of samplesrepresentative of an echo characteristic. Subsequently, the sample setsare delayed and summed within a DSP, ASIC or other circuitry to yield asingle focused Radio Frequency (RF) echo line. The simplest beamformingstrategy is to apply complex weightings to each element signal and thensum these to focus and calculate an individual image pixel. This signalforms the basis of one image line. For a C-Scan image, this one A-line(as is known in the field) is condensed to a single pixel location inthe C-Scan image. The single pixel value may be a single selected valuefrom within the A-line data—i.e. the sample corresponding to the depthof the plane of interest. Alternatively, the sample may be interpolatedfrom nearby samples or a composite (average—weighted or unweighted) ofeither pre or post envelope detected data. Generally, in the C-Scanformat it is not necessary to dynamically update the receive focusingdelays as a function of signal depth since the signal depth is small andrequired (optimal) delays change by small amounts. Nevertheless, in somesituations where the depth of image formation is significant,dynamically updated receive delays (as is well known in field) are used.The apodization weights applied to the individual element signals mayalso be dynamically updated. Apodization is the well known approach ofapplying a distribution of weighting factors to the individual elementsignals prior to summing to produce an improved beampattern—generallyhaving lower sidelobe levels.

Because only a small number of sampling operations (perhaps even one)per element is performed per transmit event, the A/D design is muchsimpler and the resultant data rate is significantly lower than for acomparable conventional prior art system. One exemplary A/D converter600 using simplified hardware is shown in FIG. 6. A digital valuegenerator (DVG) 602 is used to provide increasing digital values (e.g.,8 bits) to Digital to Analog Converter (DAC) 604. The DVG 602 may be asimple counter, a microprocessor, or other suitable circuitry. The DAC604 provides an analog voltage on output 606 proportional to the binaryinput. The output voltage is input to comparator 608, along with thesampled voltage on line 610 from S/H circuit 612. As the voltage on DACoutput 606 passes the voltage on line 610 from S/H circuit 612, thecomparator output 614 changes state, and activates register 616 to loadthe digital value from the DVG 602. The digital conversion value isprovided on A/D output 618. The A/D converter may take other forms, andFIG. 6 is merely one of many circuits known to those of skill in theart. One reason that simplified A/D converters may be used herein is dueto the additional time available to digitize the sample. In theconverter 600 for example, numerous clock cycles may be used as the DVGgenerates new digital values for conversion to an analog voltage. Thatis, the acquisition techniques described herein provide relief from theconstraints imposed by full-rate continuous sampling over the entireecho signal of interest.

Digitized data may be temporarily stored in registers in the synthesizer234, which may be implemented as a custom IC before being read out by aprogrammable digital signal processor (DSP). Alternatively, thesynthesizer 234 may also be implemented on the DSP, along with the imageprocessing.

It is also possible for the beamforming process to be applied on thereceived samples and then perform the assembly into finite lengthrecords in the post beamforming (beamsum) register. In this mode ofoperation, individual samples are delay compensated and used to populatean accumulating beamsum register (i.e. a register which adds new valuesto current values in the correct register location.) which adds inisolated samples from successive transmit events. Over the set offirings, the process will progressively populate the beamsum register,in a type of pipelining operation. (Alternatively, if the output is notfully populated, then values intermediate between populated values arefilled by interpolation between the populated values.) This process ispresented an alternative preferred embodiment. The relative merit ofvarious processing approaches is related to the relative cost of delayprocessing and memory. In addition, the time required to render an imagemay be reduced since the beamforming process may begin before all of thedata samples have been acquired. The approach used in this embodimentmay not work as well in the context of inter sample interpolation—i.e.in practice, it would require a higher sampling rate because thebeamforming approach suggested here is based on nearest neighbor delayoperations as opposed to interpolated, or phase corrected, delays.

In an alternative embodiment shown in FIG. 3, hardware is provided toacquire multiple samples per transmit event. Note that for convenience,only three receive channels are depicted. The hardware of FIG. 3 may beconfigured to obtain a single complex valued sample at each samplinginstant. As described in the related applications set forth above, agood approximation of a complex sample may be obtained by obtaining tworeal samples offset by a quarter wavelength of the center frequency ofthe transmitted waveform. Thus, each receive channel includes a pair ofS/H circuits 318/319, 320/321, 322/323, and associated A/D circuits. Thetiming circuit 336 generates two timing signals, one on line 337 havingthe desired time offset, and one on line 338 that occurs one quartercycle later.

In other embodiments, standard demodulation techniques are used toobtain complex sample values. That is, the echo signal may be formedinto inphase and quadrature components by channeling the echo signalinto two paths and mixing each of (multiplying) them with inphase andquadrature reference signals respectively. The two signals are then lowpass filtered to eliminate the sum frequency components. As is known tothose of skill in the art, the low pass filtered signals may represent abaseband version of the echo. Samples of this signal may then beacquired as complex samples described herein, and are representative ofthe magnitude and phase of the echo signal. Alternatively, the echosignal may be mixed down to an intermediate frequency (IF) and sampledas described herein. In this case, the samples may be real valued, ormay be further processed by a DSP to obtain complex samples, or the DSIQsampling technique may be applied, where the one quarter period offsetis determined by the center frequency of the IF signal. Note also thatDSIQ samples may be obtained from samples taken from different points(e.g., three quarters offset) or even different cycles of the waveform,as long as the timing of the sampling has the appropriate quadraturephase relationship. Of course, complex samples may be generated fromDSIQ samples taken at other offsets, but one-quarter or three-quarterperiod offsets (or multiples thereof) provide orthogonal measurements(resulting from the sine/cosine sinusoid decomposition) making complexsample generation more computationally efficient. In this way, it ispossible to accumulate finite time records of any of: real radiofrequency data, complex baseband data (via standard demodulation) orDSIQ data.

Note that the A/D circuits may be provided in pairs as shown by A/Dcircuits 326/327 and 328/329, or alternatively, a single A/D circuit 330may be shared by the two S/H circuits 322/323, with the sampleconversion being applied serially.

In another configuration, the hardware of FIG. 3 may be used to obtaintwo real-valued samples separated by a single sample interval. Inparticular, timing signals on line 337 and 338 may be generated to causeS/H circuits 318 and 319 to sample the receive echo waveform at twoseparate time offsets, thereby resulting in a echo sample subset of twosamples per iteration. The samples may be of consecutive samplinginstants, or may be separate by two, three, or any number of sampleintervals. In the embodiment of FIG. 3 configured to provide two samplesper subset (i.e., per iteration), the entire record may be synthesizedfrom a number of iterations equal to one half the desired record length.Thus, in this embodiment, the system is capable of performing the dataacquisition in only two iterations: one half of the samples are gatheredfrom one burst, and half of the samples are gathered on the secondburst. The sample subsets may be obtained at full rate and thenconcatenated, or the two halves may be obtained at half the desiredeffective sampling rate, and then interleaved. In other embodiments,more S/H converters and A/D converters may be added to achieve anyarbitrary number of samples per subset.

Sampling techniques that operate at a rate less than twice the highestfrequency component are understood to be sub-Nyquist rate sampling. Ofcourse, in embodiments where sub-Nyquist rate sampling is utilized(i.e., one or more intervening sampling instants are skipped during thesubset acquisition), the resulting samples are interleaved by thesynthesizer 334 to obtain the desired sample sets representative of theentire echo characteristic.

In some embodiments, improvements in the signal to noise ratio (SNR) canbe obtained by using higher sampling rates. For example, the samplingrate can be more than doubled and will still operate perfectly well inthe majority of clinical situations. Notice also that the discussion ofminimum sampling rates relates to the highest frequency present to anysignificant extent rather than the center frequency of the ultrasoundbeing used.

In an alternative embodiment, the circuitry of FIG. 2 may be utilized toobtain multiple samples per subset. In this embodiment, the S/H circuits218, 220, 222, 224, may operate two or more times per iteration.Preferably, they are operated at a rate lower than the desired effectiverate, and typically at a sub-Nyquist rate.

In one exemplary embodiment providing a frame rate of 8 frames persecond, and providing a 65×65 pixel image, and a 32×32 element array,then the DSP performs approximately 34.6 million complex multiply andadd operations per second. Since each complex operation is equal to 4real operations, the total computation cost is approximately 138.4million operations per second. This level of computation can readily beperformed by the Texas Instruments TMS320VC5416-160 which is a low powerprogrammable DSP currently selling for less than $40. The addition of asecond DSP (or faster DSPs when they are available) would enable higherframe rates, color flow imaging, and a variety of other signalprocessing applications. The systems described herein reduces overallcost by placing analog and mixed signal components in a custom IC, whileusing off-the-shelf programmable DSPs to implement rapidly evolvingbeamforming and signal processing algorithms.

In the embodiment shown in FIG. 4, the receive channels are selectivelyconnected via switches 402 to shared data conversion hardware resources,including the S/H circuitry and A/D circuitry 404/406 and 408/410. Inthis manner, the hardware may be further simplified, with the resultbeing that more iterations must be performed because samples areobtained at each iteration from only some array elements (thoseconnected to the shared resources). Similarly, the embodiment shown inFIG. 5 may be used to obtain a single complex value using sharedhardware resources, or may be configured to provide two real samples periteration. The number of S/H and A/D elements may be increased toprovide additional samples per iteration, and the A/D elements may befurther shared among individual S/H elements. When sharing hardwareresources, the number of array elements used in a given iterationdepends on how many elements are assigned to a single converter. This isalso referred to as a hardware reuse factor. For example, a hardwarereuse factor of two means that two elements share resources, and thusone half the elements are able to obtain sample subsets during any giveniteration.

As shown in FIGS. 7A-D, at each iteration only a portion, or subset, ofthe echo characteristic is sampled and stored. During the iterations,the portion of the echo characteristic that is measured and sampled isvaried by changing the relative sampling instants. That is, the timeoffset from the transmission to the respective sampling instant isvaried. The sample sets representative of the entire echo characteristicare then compiled from the multiple subsets of the ultrasonictransmissions. Thus, rather than obtaining an entire set of echo samplesneeded for image processing from each echo, the method described hereinperforms data collection iterations by transmitting an ultrasonic pulse,and receiving a pulse echo at a plurality of array elements, and at eachelement, obtaining a pulse echo sample subset at a respective timeoffset, wherein the time offset is varied. Then, for each of therelevant array elements, the sample subsets are used to compile a singlesynthesized composite pulse echo sample set that is representative of anecho characteristic at that element.

In one embodiment shown in FIG. 7A, the pulse echo sample subsetcontains a single sample. The single sample may be a real valued sample,or may be a complex valued sample obtained from two real samplesseparated by a quarter period of the echo signal. The period referred tohere corresponds to the period of the center frequency of the echosignal. Since this is time sampling example, the sampling intervalcorresponds to one quarter of a period as measured at the centerfrequency. Center frequency can be defined in a number of ways but isgenerally associated with the frequency mid way between two cutoffthresholds—e.g. the lower and upper −6 dB levels with respect to peakspectral component. Other definitions of center frequency can be used.Center frequency here generally relates to the anticipated centerfrequency after receive transduction to an electrical signal—i.e. net oftissue and transducer related spectral shaping and frequencydownshifting.

Alternatively, the subset may be a plurality of samples, as shown inFIG. 7B. A subset having a plurality of samples may be obtained byfull-rate sampling of the echo signal for a portion of the echo. In thisembodiment, each subset contains no more than half the desired samples.The multiple full-rate samples may be obtained, as described herein,from a plurality of A/D converter hardware resources, or a singlefull-rate converter that is only capable of providing a few consecutivesamples. In one preferred embodiment, each subset contains no more thansix consecutive samples.

In an alternative embodiment shown in FIG. 7C, the plurality of samplesin one subset may be obtained by sub-Nyquist rate sampling. That is,samples may be obtained from a single echo signal at rate below theNyquist rate. In subsequent iterations, samples (also at a sub-Nyquistrate) may be obtained, where the sampling instants have a different timeoffset. In both cases (successive sampling or sub-Nyquist sampling), thesubsets are then combined to obtain to a full-length sample setrepresenting the echo characteristic, sampled at an effective rate of atleast the Nyquist rate. Thus, the number of desired samples in thesample set is simply the number of iterations times the number ofsamples obtained per iteration. For example, if a desired characteristicecho sample set is length N samples, and the number of data collectioniterations is j, then the number of samples in each pulse echo samplesubset is an integer equal to approximately N/j.

For embodiments using successive sampling, the sample subsets areconcatenated into a single record for each element, whereas forembodiments using sub-Nyquist sampling, the sample subsets areinterleaved to obtain the full echo characteristic sample record. Thebeginning and ending time offsets are determined in large part by thetissue depth of the desired image.

The step of synthesizing the pulse echo sample subsets of varied offsetsinto a single composite pulse echo sample set may be performed byconcatenation or interleaving, as shown in FIGS. 7 A, B, and C. Thesynthesizing step may also include manipulating the sample subsets inresponse to at least one motion estimation value as well as therespective time offsets used to gather the data.

In this regard, one or more array elements may be used to obtain samplesused for motion estimation. A single motion estimation value may beobtained from individual direct-sampled complex echo samples taken overa plurality of iterations from a single element, each sample having thesame time offset.

Comparisons of successive captures of this complex sample may be used todetermine whether there has been significant motion. Specifically,motion may be estimated by a phase shift in the sampled data. If thereis significant motion, one preferred embodiment causes the system towait and try again in the expectation that motion will stop for asufficiently long time for a ‘good’ acquisition set to be obtained.

Alternatively, the number of samples acquired can be adjusted downwardsif there is some motion detected. In this manner, the record length maybe adaptive, in that samples are taken over a shorter time period in thepresence of motion. This corresponds to using the technique with a largef/number and hence getting a slightly inferior resolution. However,motion related problems will be mitigated. Another alternativeimplementation uses the signal acquired in common among acquisitions asa method to quantify the motion between transmissions. If the motion isfairly uniform over the tissue region being interrogated then thesamples may be manipulated to compensate for the motion, such as byrotating the phases of successive acquisitions, or by interpolating thesamples.

It is understood by those of skill in the art that propagation delaycompensation may involve delays at offsets other than an integer numberof sample offsets, and that well-known interpolation techniques (often acombination of interpolation followed by decimation) are used togenerate the required samples.

In one embodiment, the axial motion estimate may be used in thesynthesizing step. In particular, an approximation to the requiredre-interpolation could be simply achieved by skewing the timing of thetriggering of successive samples from timing sources 236/336 to providethe compensation in the pre sampling domain. In this way, apart frominitial motion estimation, all subsequent compensation does not need aninterpolation step since it is inherently ‘resampled’ by virtue of theskewed sampling used.

One example of fine tuning the triggering interval to provide theequivalent of sampling with no tissue motion is as follows: if thedetected motion estimate determines that successive echoes are 0.05microseconds closer than the previous ones, then the sampling trigger isreset to 0.05 microseconds earlier on successive captures (0.05 ms, 0.10ms, 0.15 ms, etc.). In embodiments using irregular transmit pulsespacing, the triggering can be scaled appropriately.

In an alternative embodiment shown in FIG. 7D, echo sample subsets thatoverlap in time may be gathered at each iteration. That is, the timeoffsets at each iteration are adjusted such that the relative samplinginstants have substantial overlap. In this way, the synthesized echorecord may be compiled from the subsets, and in addition, consecutiveecho sample subsets may be used to estimate motion. Motion may beestimated using any one of a number of different techniques know tothose of ordinary skill in the art, including cross-correlation,spline-based estimation, phase rotation, Fourier-based correlationmatching, zero-crossing matching, etc. This data collection mechanismmay be used at a single array element, at regional array elements, orall array elements. Furthermore, with overlapping time offsets, signalaveraging may be utilized to reduce the impact of non-coherent additivenoise.

Alternatively, multiple motion estimation values may be derived fromsamples taken from elements that are at different locations within thearray. In this manner, the motion may be evaluated for different regionsof the array to accommodate tipping of the array as might occur whenpressed against the tissue to be imaged. These may be referred to asregional motion estimation values. The regional motion estimation valuesmay then be used to generate array-element specific motion estimationvalues.

In addition, the compact imaging device preferably includes a button totrigger image capture. Preferably a delay is interposed before imagecapture is initiated so that motion possibly associated with thepressing of the button will have ceased (typically after 200 to 500milliseconds).

However obtained, the motion estimation value(s) may be used toenable/disable image processing to prevent the display of distortedimages caused by excessive motion. Alternatively, the motion estimationvalue(s) may be used to alter or manipulate the data samples whenforming the synthesized echo response characteristic in order tocompensate for the motion.

One preferred method 800 is described with respect to FIG. 8. At step802, an ultrasonic transmit array is used fire ultrasonic waveforms fromthe transducer elements; after each transmit firing, at step 804 asubset of echo samples of an ultrasonic echo is obtained at each of aplurality of transducer array elements. Each subset is a portion of adesired echo response as determined by a time offset. The transmitfiring and subset sampling is iteratively performed until all thedesired samples have been obtained, as determined by block 806. At step808, an equivalent full set of echo samples is retrospectivelyformulated from the subsets for each of the plurality of transducerarray elements.

The devices and methods described herein provide very good resolutionC-Mode (or B-Mode if desired) imaging when there is no tissue motion.However, when considering shallow C-Mode imaging, the ability to operateat a higher pulse repetition rate than is normally used in deep tissueB-Mode imaging provides for faster data acquisition, minimizing theeffects of motion. Hence, relatively high frequencies withcorrespondingly rapid signal attenuation are desired. Additionally,since C-mode imaging involves forming images at a single depth,‘reverberation’ (multi-path) artifacts from previous firings arestatistically less likely to inject themselves within the reconstructedimage plane than is the case in B-Mode imaging.

In some preferred embodiments, the transmit pulses may occur atnon-uniform intervals. To obtain the desired sample subset, it is onlynecessary that the receive trigger is locked to the transmit event. Itis not required that the transmit event follows a simple pattern. Anon-uniform pulse pattern has the benefit of minimizing secondary echoartifacts. That is, residual echoes from previous transmit pulses thatcontribute to the measured echo from the present pulse will tend tocancel out in the later beamforming processing steps. Thus, differentpulse intervals help make residual echoes non-coherent over successivefirings. The pulse intervals are preferably varied in a random orpseudorandom range of up to ±20%.

The complete system need only be slightly larger than the transduceritself, allowing placement of an LCD (or other flat, low profile)display directly over the transducer. This will facilitate familiarityfor new users and improve the utility of ultrasound for guiding invasiveprocedures such as catheter insertion.

One application for the techniques described herein is for guidingneedles for venous access. A secondary application is for biopsyguidance. In each case, the application generally involves a shallowimaging depth. The roundtrip propagation time through tissue to 20 mm is26 microseconds (μs). Transmit pulses and sample acquisition ispreferably performed at a rate of one per 40 μs. In most diagnosticultrasound the line firing rate is significantly slower than this sincethe imaging depth is generally greater, and the probability ofartifactual residual echo signals being superimposed within thereconstructed image are greater. Furthermore, it is desired that thehighest practical frequency be used in any clinically usable product andthat the intensity of the transmitted signal be attenuated if requiredto balance the need for SNR at the focal region of interest versus thedesire to dissipate ultrasound signals potentially arising from deeperreflectors in the tissue, minimize transducer heating and prolongingbattery life.

f/1.1 imaging (i.e. the entire aperture in this example) is used toobtain the highest possible resolution. This implies a maximum pathdifference for focusing of 1.93 mm. (for embodiments using receive-onlybeamforming only one-way path differences are used). At a speed of soundof 1540 m/s the required focal delay difference between the center ofthe array and the farthest active edge of the array is 1.25 μs. At anoperating frequency of 5 MHz, this corresponds to just over 6 periods.Preferably, additional samples are taken to fully encompass the RF pulseassociated with the finite bandwidth echo from a single idealized pointtarget. For example, a transducer pulse echo signal −6 dB fractionalbandwidth of approximately 30% may be used, and thus may addapproximately 4 cycles of acquisition time to fully encompass theanticipated pulse and to provide for slice thickness integration in thefinal image. Thus, approximately 10 cycles worth of data are required toyield data to enable a conventional delay and sum beamforming operationfor a single C-Mode plane.

In preferred embodiments, the effective sampling rate is 40 MS/s, whichis significantly higher than required by the Nyquist criterion. Thus, 80samples are required per data record. If each transmit/receive firingoccupies 40 μs, the time duration for 80 samples to be acquired usingthe current approach is 3.2 ms. Clearly, it is also possible to operatecloser to the Nyquist limit and thereby reduce the total acquisitiontime and reduce tissue motion effects.

Motion of the transducer relative to static tissue may be restrained toless than 1 mm/s in the beam axis dimension. Thus, in the time it takesto acquire a complete set of 80 samples, total target motion is 3.2 μmwhich is significantly less than a wavelength (0.31 mm). Even when 10mm/s motion is present, and a longer time per acquisition is used (e.g.80 μs), motion is still small compared with realistic wavelengths. TheSAA approach may not be suitable for tissue regions in which there is aconsiderable motion—such as moving blood.

The parameters discussed above are set forth in Table 1.

TABLE 1 System Parameters Center Frequency 5 MHz Fractional −6 dB 30%bandwidth f/#  1.1 Number of Elements 60 Element center-center 0.3 mmpitch Apodization Quartic Root Hann Target Idealized wire at 20 mm depth

The Quartic Root Hann filter is used so as to ‘fatten’ the aperture,with respect to that obtained using a simple Hann window, and thusslightly tighten main lobe resolution and increase SNR at the expense ofincreased sidelobe level.

Exemplary embodiments of the invention have been described above. Thoseskilled in the art will appreciate that changes may be made to theembodiments described without departing from the true spirit and scopeof the invention as defined by the claims.

1. A method of obtaining a plurality of pulse echo sample sets at aplurality of ultrasonic transducer array elements, each pulse echosample set being representative of an echo characteristic for use inultrasound imaging using propagation delay compensation signalprocessing; the steps comprising: (i) performing data collectioniterations by transmitting an ultrasonic pulse, and at each iteration:(a) receiving a pulse echo at a plurality of array elements, and (b) ateach of the plurality of array elements, obtaining a pulse echo samplesubset at a respective time offset, wherein the time offset is variedbetween successive iterations; and (ii) for each of the plurality ofarray elements, aggregating the pulse echo sample subsets of variedoffsets into a single synthesized composite pulse echo sample set thatis representative of an echo signal.
 2. The method of claim 1 whereinthe pulse echo sample subset is a single sample.
 3. The method of claim1 wherein the pulse echo sample subset is a single complex sample. 4.The method of claim 1 wherein the pulse echo sample subset contains anumber of samples less than or equal to one half the number of samplesin the pulse echo sample set.
 5. The method of claim 1 wherein eachpulse echo sample set is an integer N samples long, and the number ofdata collection iterations is j, and wherein the number of samples ineach pulse echo sample subset is an integer equal to approximately N/j.6. The method of claim 1 wherein the step of obtaining the pulse echosample subset is performed by sampling the pulse echo at a sub-Nyquistsampling rate, and wherein the step of aggregating the pulse echo samplesubsets is performed by interleaving the samples from the pulse echosample subsets.
 7. The method of claim 1 wherein the plurality of arrayelements is substantially all the elements in the array.
 8. The methodof claim 1 wherein the pulse echo sample subsets are obtained usingshared sampling hardware resources, and wherein the plurality of arrayelements are a fraction of all the elements in the array, wherein thefraction is determined by a hardware reuse factor.
 9. The method ofclaim 1 wherein at least one motion-detection sample is obtained at eachiteration.
 10. The method of claim 1 wherein the smallest time offset isdetermined by a desired target depth.
 11. The method of claim 1 whereinthe step of aggregating the pulse echo sample subsets into a singlesynthesized composite pulse echo sample set comprises manipulating thesample subsets in response to at least one motion estimation value andthe respective time offsets.
 12. The method of claim 11 wherein the atleast one motion estimation value is obtained from a plurality ofindividual direct-sampled complex echo samples taken over a plurality ofiterations, having the same time offset.
 13. The method of claim 1wherein at each of the plurality of array elements, the pulse echosample subsets at consecutive iterations overlap in time.
 14. The methodof claim 13 wherein the pulse echo sample subsets from consecutiveiterations are used to generate at least one motion estimation value.15. The method of claim 1 further comprising the step of determining atleast one motion estimation value.
 16. The method of claim 15 whereinthe at least one motion estimation value comprises a plurality ofregional motion estimation values for different regions of the array.17. The method of claim 16 wherein the regional motion estimation valuesare used to generate array-element specific motion estimation values.18. The method of claim 1 wherein the time offsets used for each elementare varied in accordance with collecting data for producing an image ofa plane that is non-parallel to the transducer array.
 19. The method ofclaim 1 wherein the step of transmitting an ultrasonic pulse comprisestransmitting a plane wave using all the transducer elements.
 20. Themethod of claim 1 wherein the step of transmitting an ultrasonic pulsecomprises transmitting a plane wave using a subset of the transducerelements.
 21. The method of claim 1 wherein the step of transmitting anultrasonic pulse comprises using at least a subset of elementstransmitting to a selected focal zone
 22. The method of claim 1 whereinthe time interval between successive transmit pulses is varied for atleast some iterations.
 23. A method collecting ultrasonic echo samplesfor use in ultrasonic image processing, comprising the steps of:performing multiple transmit firings of ultrasonic waveforms from anultrasonic transducer array of transducer elements; after each transmitfiring, obtaining a subset of echo samples of an ultrasonic echo at eachof a plurality of transducer array elements, wherein each subset is aportion of a desired echo signal as determined by a time offset;retrospectively formulating an equivalent full set of echo samples fromthe subsets for each of the plurality of transducer array elements. 24.The method of claim 23 wherein the subset of echo samples are obtainedby sampling the ultrasonic echo at a sub-Nyquist sampling rate.
 25. Themethod claim 23 wherein the step of retrospectively formulating anequivalent full set of echo samples is performed in response to a motionestimation value.
 26. The method claim 23 wherein the step ofretrospectively formulating an equivalent full set of echo samples fromthe subsets is performed by concatenating samples from the subsets fromthe respective elements.
 27. The method claim 23 wherein the step ofretrospectively formulating an equivalent full set of echo samples fromthe subsets is performed by interleaving samples from the subsets fromthe respective elements.
 28. The method claim 23 wherein the time offsetis adjusted in response to at least one motion estimation value.
 29. Themethod claim 23 wherein a time interval between the multiple transmitfirings is not constant.
 30. A method collecting ultrasonic echo samplesfor use in ultrasonic image processing, comprising the steps of:performing multiple transmit firings of ultrasonic waveforms from anultrasonic transducer array of transducer elements; at each of aplurality of transducer array elements, obtaining a plurality of samplesubsets of echo samples, wherein each subset is a portion of a desiredecho signal; for each of the plurality of transducer array elements,synthesizing an equivalent full set of echo samples at a desiredeffective sampling rate from each of the plurality of subsets.